The present invention relates to a hydraulic fluid pressure control system for use with a power assist steering, and more particularly to a hydraulic fluid pressure control system of the above kind which can hydraulically activated not only a hydraulically operated device, i.e., a power cylinder, of a power assist steering for dirigible wheels of a vehicle, but also some other hydraulically operated device on the vehicle.
In automotive vehicles, a hydraulic fluid pressure control system which hydraulically activates not only a power cylinder of a power assist steering system for a pair of front dirigible wheels, but also a rear power cylinder in a rear suspension for varying the angle that a pair of rear wheels would otherwise assume during cornering so as to compensate for the so called "compliance steering effect" induced in the rear suspension. In one such system disclosed, for example, in U.S. Pat. No. 4,440,254 issued to Sibahata et al. (corresponding to German Patent Application No. P3172164.8), a compliance steer control system is responsive to pressurization of right and left cylinder chambers of a power cylinder of a front power steering system to produce a bias which tends to turn the pair of rear wheels in response to steering torque. This known system is advantageous in that, with a single power steering control valve, both the power assistance to the front dirigible wheels and the bias to turn the rear wheels are varied in response to steering torque applied to the control valve. However, with this known system, it is not possible to give different output vs., steering torque input characteristics which are required by the power assist steering and the compliance steer control, respectively steer control, respectively. Thus, in another improved system as disclosed in U.S. Pat. No. 4,586,581 issued to Shibahata et al. (corresponding to DE-OS 34 37 071), in addition to a pump, and a power steering control valve designed for use in a power assist steering for front dirigible wheels, a second pump and a second control valve are provided for a compliance steer control of rear wheels. In this known system, the discharge rate of hydraulic fluid from the first pump is high at low vehicle speeds to provide a high degree of power assistance, but is decreased to cause a reduction in the degree of power assistance. The setting of the second pump is different from the first pump such that the discharge rate of hydraulic fluid is very low or almost zero at low vehicle speeds, but increases up to the miaximum at high vehicle speeds. This known system requires two pumps, two control valves and two sets of pipings, thus posing the following problems, thus causing cost increase, making it difficult to install on the vehicle, and increasing burden beared by an engine driving these pumps.
An object of the present invention, therefore, is to provide a hydraulic fluid pressure control system of the above kind which, with a single control valve, two different hydraulic fluid pressure output variation characteristics are given versus a predetermined variavle, such as a steering torque.